Pressurized oxygen for evaluation of molding compound stability in semiconductor packaging

ABSTRACT

A test environment and an associated method of testing and analyzing a semiconductor package material containing a molding compound, for stability in a sustained oxygen environment. Test samples are exposed to a pressurized gas containing oxygen, under elevated temperature below the glass transition temperature of the molding compound. Control samples are exposed to a pressurized inert gas under similar or more severe conditions of gas pressure, temperature, and humidity. At least one characteristic common to the test samples and the control samples is measured. A determination is made as to whether there exists at least one significant difference between the at least one measured characteristic of the test samples and the control samples.

This application is a continuation application claiming priority to Ser.No. 11/861,369, filed Sep. 26, 2007, which is a continuation of U.S.Pat. No. 7,300,796, issued Nov. 27, 2007.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a test environment and an associatedmethod of testing and analyzing a semiconductor package material forstability in a sustained oxygen environment.

2. Related Art

A serious industry-wide problem is known to exist with semiconductorpackages having an epoxy molding compound that includes an elemental redphosphorus-based flame retardant additive. After about two and a halfyears in the field, some of the semiconductor packages and materialstherein failed, such as by developing electrical shorts, particularlyduring summer months during periods of high temperature and humidity. Inefforts to understand why the fails occurred, the packaging materialwith red phosphorus was subjected to extensive testing by variousinterested parties, including conventional accelerated stress tests,tests involving exposure to humidity, elevated temperature, and voltage,etc. Unfortunately, the preceding tests demonstrated acceptableperformance and therefore did not provide any insight as to why theaforementioned fails have been occurring with the semiconductorpackages. The nature of this problem and the lack of insight into whythis problem exits, as well as adverse consequences of this problemincluding the filing of multiple lawsuits, are discussed in “NikkeiElectronics” (Oct. 21, 2002). At present, there is no publicly availableinformation that sheds light as to why the aforementioned fails havebeen occurring.

Accordingly, there is a need for a method of testing and analysis, aswell as an associated test environment, that leads to detection andminimization of potential failure mechanisms in semiconductor packagescontaining potentially unstable materials such as red phosphorus.

SUMMARY OF THE INVENTION

The present invention provides a method of testing a semiconductorpackaging material containing a molding compound for stability of thesemiconductor packaging material in a sustained oxygen environment, saidmethod comprising:

providing N substantially identical samples such that N is a positiveinteger of at least 2, wherein each of the N samples comprises thesemiconductor packaging material, wherein T samples of the N samples aretest samples, wherein C samples of the N samples are control samples,and wherein T and C are positive integers such that T+C=N;

exposing during a time period τ the T test samples to a pressurized gashaving a total pressure P_(TOT)(t), said pressurized gas comprisingoxygen gas, wherein for times t during 0≦t≦τ the oxygen gas has apartial pressure P(t) of at least P₁ and a temperature T(t) satisfyingT_(G)−ΔT₂≦T(t)≦T_(G)−ΔT₁ such that 0≦ΔT₁≦ΔT₂ for glass transitiontemperature T_(G) of the molding compound, wherein during 0≦t≦τ the Ttest samples are exposed to moisture having a relative humidity H(t)such that H₁≦H(t)≦H₂, wherein H₁≧0% and H₂≦100%, wherein τ is at leastabout 12 hours, wherein P₁ is about 15 psi, and wherein T_(G)−ΔT₂ is atleast about 20° C.; and

exposing the C control samples during times t for a time period τ′ to apressurized inert gas having a pressure P′(t) and a temperature of T′(t)at a relative humidity H′(t), wherein a common time interval exists fortimes t during which both the pressurized gas comprising oxygen and thepressurized inert gas are being exposed by the respective exposingsteps, wherein during said common time interval: P′(t)≧P(t) or P′(t)does not substantially differ from P(t), T′(t)≧(T(t) or T′(t) does notsubstantially differ from T(t), and H′(t)≧H(t) or H′(t) does notsubstantially differ from H(t).

The method may further comprise (after said exposing the T test samplesand the C control samples):

measuring at least one characteristic common to the C control samplesand the T test samples; and

determining whether there exists at least one significant differencebetween the at least one measured characteristic of the T test samplesand the at least one characteristic of the C control samples.

The present invention provides a test environment, comprising a chambercontaining S samples, a pressurized gas, and moisture, wherein the Ssamples each comprise a semiconductor packaging material that includes amolding compound, wherein S is at least 1 and if S>1 then the S samplesare substantially identical, wherein the S samples are being exposed tothe pressurized gas and the moisture, wherein the pressurized gasincludes at least one of oxygen gas and an inert gas, wherein thepressurized gas has a temperature T satisfying T_(G)−ΔT₂≦T≦T_(G)−ΔT₁such that 0<ΔT₁≦ΔT₂ for glass transition temperature T_(G) of themolding compound, wherein the moisture has a relative humidity H suchthat H₁≦H≦H₂, wherein H₁≧0% and H₂≦100%, wherein T_(G)−ΔT₂ is at leastabout 20° C., wherein if the pressurized gas includes the oxygen gasthen the partial pressure of the oxygen gas is at least about 15 psi,and wherein if the pressurized gas does not include the oxygen gas thenthe pressure of the inert gas is at least about 15 psi.

The present invention provides a method of testing and analysis, as wellas an associated test environment, that prevents or reduces failsrelating to semiconductor packages having a molding compound materialthat contains red phosphorus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table depicting phosphate and chloride ion extractionconcentrations from semiconductor packages that had been subjected topressurized oxygen and pressurized nitrogen, in accordance withembodiments of the present invention.

FIGS. 2A and 2B are thermogravimetric graphical profiles forsemiconductor packages that had been subjected to pressurized oxygen andpressurized nitrogen, in accordance with embodiments of the presentinvention.

FIG. 3 is a flow chart depicting a method of testing and analyzing asemiconductor package material for stability in a sustained oxygenenvironment, in accordance with embodiments of the present invention.

FIG. 4 depicts a test chamber containing semiconductor package samplesand a pressurized gas, in accordance with embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

As explained supra in the Related Art section, there is no publiclyavailable information that sheds light as to why the fails, includingelectrical shorts, have been occurring with semiconductor packages whichinclude molding compound material containing red phosphorus. A moldingcompound of a semiconductor package is defined herein, including in theclaims, as a thermosetting plastic for packaging of semiconductors.Properties of the molding compound include suitable dielectric strength,arc resistance, dry insulation resistance, low dielectric constant,dimensional stability, low flammability, and ease of molding. Examplesof molding compound classes that may exist in semiconductor packagesinclude alkyds, aminos, diallyl phthalates, epoxies, fluoropolymers,phenolics, polyesters and rigid silicones.

In response to this problem, the inventors of the present invention haveperformed experiments that explain the root cause of this problem. Basedon the results of said experiments, the inventors of the presentinvention have formulated a test and analysis methodology, and anassociated test environment, for preventing or reducing adverseconsequences (e.g., electrical shorts) associated with this problem.Accordingly, said experiments will be next described, followed by adescription of the test and analysis methodology, and an associated testenvironment, of the present invention.

Experiments

The inventors of the present invention hypothesized that oxygen could bethe root cause of the problem, through interaction between oxygen andphosphorus in the presence of moisture. The present inventors testedthis hypothesis, by subjecting the packaging material to a 100%pressurized oxygen environment at elevated temperature and humidity toaccelerate the reaction. In particular, the tests were performed at 1800psi oxygen pressure at 85° C. (which is below the glass transitiontemperature of the molding compound of the packaging material) for atleast 4 days, which provides acceleration of about 600 times the normalreaction with oxygen at room temperature and atmosphere concentrationsof oxygen. These results showed that the packaging material broke down,and large quantities of phosphates were extractable from the moldingcompound following the testing. The experimental results demonstratedthat the oxygen reacted with phosphorus in the presence of moisture togenerate phosphoric acid. Follow-up experimentation (i.e., electricaltesting, material property testing, thermogravimetric analysis) showedthat the properties of the molding compound had changed and thatelectrical shorts were generated. The experimental results correlatedwith observations of similar or comparable packaging materials in thefield (i.e., semiconductor package types that failed in the field alsotended to fail in the experiments, whereas semiconductor package typesthat did not fail in the field also tended not to fail in theexperiments). The experimental results support a mechanism of phosphoricacid generation, wherein the generated phosphoric acid provides anelectrolyte path for the migration of copper ions under an appliedvoltage bias, and wherein the copper ions result from the dissolving ofcopper in the phosphoric acid. Thus the electrical shorts were caused bythe migration of the copper ions in the phosphoric acid generated by thechemical reaction of phosphorus and oxygen in the presence of moisture.

FIGS. 1 and 2, which summarize a portion of the experimental results,are next described.

FIG. 1 is a table that lists ion extracts of phosphate ions and chlorideions, obtained from ion chromatography of ten different semiconductorpackages (identified in the “Package Type”) which were each subjected topressurized oxygen at 1800 psi partial pressure of oxygen for 4 days ata temperature of 85° C. subject to moisture contact at a relativehumidity of 100%. From 1 to 8 samples were used for each semiconductorpackage type. During the exposure to pressurized oxygen, the packageswere in an open vial in a closed, heated chamber. Following the exposureto the pressurized oxygen, about 25 milliliters of deionized water wasadded to the vial and the packages were soaked in the deionized waterfor 24 hours; then the deionized water was analyzed by ionchromatography to determine the type and concentration of ions extractedfrom the package into the deionized water.

The temperature of 85° C. is high enough to accelerate the reaction withoxygen, but is less than the glass transition temperature of the moldingcompound that is within any of the ten packages tested, in order toreduce or minimize the chance of stress relief from material propertiesthat exist in the molding compound at T_(G) or above.

The ten different semiconductor packages were also subject topressurized nitrogen as a control, at the same gas pressure,temperature, and moisture content as existed with the oxygen gas. One ormore samples were used for each package type. Thus, FIG. 1 has an“Oxygen Gas” column and a “Nitrogen Gas” column for both the phosphateion extract data and the chloride ion extract data. The ion extractdata, averaged over the samples for each package type, is expressed inunits of micrograms per square millimeter of package surface. The “FieldIssues?” column identified whether electrical fails (i.e., shorts) hadbeen observed in the field for each type of module (i.e., “yes” meansthat electrical fails have been observed, while “no” means thatelectrical fails have not been observed). Package types 1-5 included amolding compound comprising red phosphorus, while package types 6-10included a molding compound not containing red phosphorus but containingphosphorus in the form of an organic phosphorus compound.

FIG. 1 shows that negligible amounts of phosphate ions and chloride ionswere extracted when pressurized nitrogen gas was used, which is asexpected since nitrogen gas is inert and is therefore not expected tochemically react with any packaging material.

The extracted phosphate shown in FIG. 1 is significantly higher for thepressurized oxygen environment than for the pressurized nitrogenenvironment for packages 1-5, which correlates with the fact thatpackages 1-5 each have a red phosphorus-containing molding compound andalso with the fact that package types 1-5 have known or suspected fieldissues. The extracted phosphate has a concentration ranging from 0.15 to2.5 μg/mm² for packages 1-5, which correlates with the fact thatpackages known to have had problems in the field had ion concentrationsof the order of 0.1 μg/min² or higher. Thus, a measured phosphateconcentration of at least 0.1 μg/mM² (or a more conservative phosphateconcentration of at least 0.05 μg/mm²) may be a useful threshold fordetection of an oxygen reaction problem. In contrast, packages 6-10 showno statistically significant difference in extracted phosphate for thepressurized oxygen environment as compared with the pressurized nitrogenenvironment, which correlates with the fact that packages 6-10 each lackred phosphorus (but contain phosphorus bound in an organic phosphoruscompound) and also with the fact that package types 6-10 have no knownfield issues.

The extracted chloride shown in FIG. 1 is significantly higher for thepressurized oxygen environment than for the pressurized nitrogenenvironment for packages 1-5, which again correlates with the fact thatpackages 1-5 each have a red phosphorus-comprising molding compound andalso with the fact that package types 1-5 have known or suspected fieldissues. The extracted chloride has a concentration ranging from 0.01 to0.07 μg/mm² for packages 1-5. In contrast, packages 6-10 show nosignificant difference in extracted chloride for the pressurized oxygenenvironment as compared with the pressurized nitrogen environment, whichagain correlates with the fact that packages 6-10 each have a redphosphorus-free molding compound and also with the fact that packagetypes 6-10 have no known field issues. Although there is no expectedchemical reaction involving chloride, increased porosity of packagingmaterial resulting from the reactions between oxygen and phosphorusprovides migration paths for chloride ions already present in thesemiconductor packages, which accounts for the aforementioned chlorideion concentrations detected by the ion chromatography.

FIG. 2A depicts a thermogravimetric profile of package #2 of FIG. 1,while FIG. 2B depicts a thermogravimetric profile of package #9 ofFIG. 1. The package of FIG. 2A contained inorganic red phosphorus (as inthe table of FIG. 1, #2), while the package of FIG. 2B did not containred phosphorus but contained phosphorus in a bound form in organicphosphorus compounds (FIG. 1 #9). In both FIGS. 2A and 2B, the packagesample was subjected to a gas pressure of 1800 psi and a temperature of85° C. for 4 days. The relative humidity was 100% in FIG. 2A and 100% inFIG. 2B. Results are shown for both pressurized oxygen gas andpressurized nitrogen gas. The nitrogen gas serves as a control. Thethermogravimetric profile shows weight of sample versus temperature overa temperature range of 30° C. to 400° C. In FIG. 2A, thethermogravimetric profile is statistically different for the pressurizedoxygen as compared with the pressurized nitrogen. For example in FIG.2A, at 300° C. the weight reduction is 1.08% for the pressurized oxygenand only 0.34% for the pressurized nitrogen. In FIG. 2B, there is nostatistically significant difference between the thermogravimetricprofiles for the pressurized oxygen and the pressurized nitrogen. Forexample in FIG. 2B, at 300° C. the weight reduction is 0.43% for thepressurized oxygen and 0.37% for the pressurized nitrogen.

The preceding experimental results demonstrate that the pressurizedoxygen reacted with phosphorus in the presence of moisture to generatephosphoric acid. Follow-up testing (electrical testing, materialproperty testing, thermogravimetric analysis) showed that the propertiesof the molding compound had changed and that electrical shorts weregenerated. The electrical shorts were generated indirectly as aconsequence of the changes in material properties, such as expansion ofthe material or creation of cracks in the material. In particular, theelectrical shorts were generated through an electrolyte path asexplained supra.

Based on the preceding experimental results, a method of testing andanalysis of semiconductor packages subjected to a pressurized oxygen,with nitrogen or another inert gas serving as a control, may be used toassess the stability of such semiconductor packages in a sustainedoxygen environment, as described next.

Test Environment, Methodology, and Analysis of the Present Invention

FIG. 3 is a flow chart depicting steps 61-66 which describe a process ormethod of testing and analyzing a semiconductor package material forstability in a sustained oxygen environment, in accordance withembodiments of the present invention. The stability includes mechanicalstability, electrical stability, and chemical stability.

Step 61 provides one or more substantially identical test samples, eachcomprising the semiconductor package material containing a moldingcompound. The test samples will be subjected to pressurized oxygen, aswill be described infra in conjunction with step 62. Step 61 alsoprovides one or more substantially identical control samples, eachcomprising the semiconductor package material. The control samples willbe subjected to a pressurized inert gas (e.g., nitrogen, argon, etc.),as will be described infra in conjunction with step 63. Each of the testsamples is substantially identical to each of the control samples. Twosamples are said to be substantially identical if the two samples differonly in a minor respect which cannot be reasonably expected to impactthe stability of the semiconductor package material.

The preceding description of step 61 may be more abstractly described asproviding N substantially identical samples such that N is a positiveinteger of at least 2. Each of the N samples comprises the semiconductorpackaging material containing a molding compound. T samples of the Nsamples are test samples and C samples of the N samples are controlsamples, wherein T and C are positive integers such that T+C=N. Forexample, if T=10 and C=2 then N=12; i.e., there are 12 substantiallyidentical samples of which 10 are test samples and 2 are controlsamples. In an embodiment, the N samples are essentially identical,which is a special case of the N samples being substantially identical.

In an embodiment, the N samples each comprise a semiconductor packagethat includes the semiconductor packaging material. The semiconductorpackage material includes a molding compound. In another embodiment, theN samples each comprise a portion of a semiconductor package, whereinthe portion is less than the entire semiconductor package, and whereinthe portion includes the semiconductor packaging material. As anexample, the portion may consist of one or more cured raw materialscontaining the molding compound. In yet another embodiment, the Nsamples each comprise a coupon of the one or more cured raw materialscontaining the molding compound.

Inasmuch as the experimentation described supra relates to moldingcompounds containing phosphorus (i.e., red phosphorus), the moldingcompound in the semiconductor package material tested and analyzed inaccordance with the present invention may include phosphorus (e.g., redphosphorus). However, the scope of the present invention includes anymolding compound found in semiconductor packaging materials, includingmolding compounds not containing phosphorus.

In step 62, the T test samples are exposed to a pressurized gas during atime period τ, wherein τ is at least about 12 hours. τ may have anyvalue of at least 12 hours such as, inter alia, D days plus H hours+Mminutes+S seconds, wherein D is a non-negative integer, wherein H is apositive integer less than 24, M is a positive integer less than 60, andS is a positive integer less than 60 (e.g., D=0, 1, 2, 3, 4, >4, etc.;H=1, 2, 4, 8, 12, >12, etc; M=10, 25, 40, >40, etc.; S=10, 25, 40, ≧40etc.). If D=0 then H must be at least about 12.

The pressurized gas includes oxygen gas and may also include one or moreinert gases. In an embodiment, the pressurized gas is about 100% oxygengas. For times t during 0≦t≦τ, the oxygen gas has a partial pressureP(t) of at least P₁, wherein P₁ about 15 psi. Generally, the pressurizedgas has a total pressure P_(TOT)(t) such that P_(TOT)(t)≧P(t). Invarious embodiments, P₁ is at least about 15, 100, 300, 500, 1000, etc.In various embodiments, P(t) and/or P_(TOT)(t) may not exceed about 1500psi, 1800 psi, 2000 psi, 2500 psi, 3000 psi >3000 psi, etc. P(t) and/orP_(TOT)(t) may be approximately constant (e.g., P₂ is about equal to P₁if P(t) is approximately constant). Alternatively P(t)and/or P_(TOT)(t)may be variable in time (i.e., dP(t)/dt≠0 and/or dP_(TOT)(t)/dt≠0 within0≦t≦τ) with any time dependence (e.g., monotonically increasing,monotonically decreasing, oscillatory, etc.).

Also during 0≦t≦τ, the oxygen gas has a temperature T(t) satisfyingT_(G)−ΔT₂≦T(t)≦T_(G)−ΔT₁. ΔT₁ and ΔT₂ satisfy 0<ΔT₁≦ΔT₂ for glasstransition temperature T_(G) of the molding compound. The minimumtemperature T_(G)−ΔT₂ of T(t) is at least about 20° C. In variousembodiments, T_(G)−ΔT₂ is at least about 20° C., 35° C., 50° C., 75° C.,85° C., ≧85° C., etc. T(t) is constrained to be below T_(G) in order toreduce or minimize the chance of stress relief from material propertiesthat exist in the molding compound at T_(G) or above. T(t) may beapproximately constant (i.e., ΔT₁ is about equal to ΔT₂). AlternativelyT(t) may be variable in time (i.e., dT(t)/dt≠0 within 0≦t≦τ) with anytime dependence (e.g., monotonically increasing, monotonicallydecreasing, oscillatory, etc.).

Moisture is typically present under normal operating conditions and isthus made available during the oxygen exposure of the T test samples toreact with any reaction products that might be produced during theoxygen exposure. Accordingly, during 0≦t≦τ, the T test samples areexposed to moisture having a relative humidity H(t) such thatH₁≦H(t)≦H₂, wherein H₁≧0% and H₂≦100%. H(t) may be approximatelyconstant (i.e., H₂ is about equal to H₁) (e.g., H(t) is about equal to0%, 25%, 50%, 80%, 100%, etc). Alternatively H(t) may be variable intime (i.e., dH(t)/dt≠0 within 0≦t≦τ) with any time dependence (e.g.,monotonically increasing, monotonically decreasing, oscillatory, etc.).

During the exposure to the pressurized oxygen, the T test samples are inan open vial and the vial as well as the pressurized oxygen are within aclosed, heated chamber as is described infra in conjunction with FIG. 4.

Subtle oxygen reactions can change the properties of the moldingcompound over time and could result in failure of the semiconductorpackage as explained supra. Accordingly, a purpose of using a higheroxygen partial pressure and temperature for the testing of the presentinvention than the oxygen partial pressure and temperature existingunder ambient atmospheric conditions is to accelerate any instabilityeffects in the molding compound that might occur as a consequence ofprolonged exposure to oxygen.

In step 63, the C control samples are exposed during times t for a timeperiod τ′. to a pressurized inert gas (e.g., nitrogen, argon, etc.)having a pressure P′(t) and a temperature of T′(t) at a relativehumidity H′(t). The C control samples are not exposed to oxygen gaswhile being exposed to the pressurized inert gas. A common time intervalexists for times t during which both the pressurized gas comprisingoxygen and the pressurized inert gas are being exposed by the respectiveexposing steps. During said common time interval: P′(t)≧P(t) or P′(t)does not substantially differ from P(t), T′(t)≧(T(t) or T′(t) does notsubstantially differ from T(t), and H′(t)≧H(t) or H′(t) does notsubstantially differ from H(t).

As an alternative embodiment, the condition of “P′(t) does notsubstantially differ from P(t)” may be replaced by the more stringentcondition of “P′(t)≧P_(TOT)(t) or P′(t) does not substantially differfrom P_(TOT)(t)” (e.g., in situations in which the samples may beadversely affected by total pressure).

To be explained next is the meaning herein, including in the claims, of“does not substantially differ from” in the following expressions:“P′(t) does not substantially differ from P(t)”, “T′(t) does notsubstantially differ from T(t)”, “H′(t) does not substantially differfrom H(t)”.

Consider the expression: “P′(t) does not substantially differ fromP(t)”. To determine whether P′(t) differs substantially from P(t), onemust analyze P(t) and P′(t) comparatively as pressure profiles versustime over the entire time domains of τ and τ′, respectively. Theanalysis is not based exclusively on a comparison between P(t) and P′(t)at each point in time. For example, consider the T test samples and theC control samples being exposed for about 5 days to pressurized oxygengas at 1800 psi of pressure and to the pressurized inert gas at 1800psi, respectively, except that during 5 minutes of the 5 days ofexposure the pressure P′(t) of the pressurized inert gas is ramped downto about 300 psi and then ramped back up to about 1800 psi. In saidexample, the pressures P(t) and P′(t) do not differ substantially fromeach other because 5 minutes is only about 0.07% of the 5 days, and saiddeviation between P(t) and P′(t) is insignificant (e.g., said deviationbetween P(t) and P′(t) has no more than a negligible effect on thedifference in properties between the T test samples and the C controlsamples at the end of the exposure periods τ and τ′).

In summary, the analysis should include looking at P(t) and P′(t) (oralternatively P_(TOT)(t) and P′(t) as explained supra) at each point intime, and if P(t) and P′(t) (or P_(TOT)(t) and P′(t)) do notsubstantially differ at each point in time, then P′(t) is said to notsubstantially differ from P(t) (or from P_(TOT)(t)). However, if P(t)and P′(t) (or P_(TOT)(t) and P′(t)) substantially differ from each otherat one or more points in time, then the analysis should determinewhether said deviation between P(t) and P′(t) (or between P_(TOT)(t) andP′(t)) is insignificant as in the preceding example having the 5 minutesof deviation in P′(t).

The preceding discussion of the meaning of “does not substantiallydiffer from” in the expression of “P′(t) does not substantially differfrom P(t)” applies analogously to the meaning of the expressions “P′(t)does not substantially differ from P_(TOT)(t)”, “T′(t) does notsubstantially differ from T(t)” and “H′(t) does not substantially differfrom H(t)”.

During the exposure to the pressurized inert gas, the C control samplesare in an open vial and the vial as well as the pressurized inert gasare within a closed, heated chamber as is described infra in conjunctionwith FIG. 4.

Consider next a comparison between the exposure period τ of the T testsamples to the pressurized gas (including oxygen) and the exposureperiod τ′ of the C control samples to the pressurized inert gas.Consider the following useful embodiments: τ′ is about equal to τ, τ′does not substantially differ from τ, τ′<τ, and τ′>τ.

The embodiment of “τ′ is about equal to τ” is characterized by a strictlevel of experimental control for utilizing about the same exposureperiod for the C control samples as for the T test samples.

The embodiment of “τ′ does not substantially differ from τ” ischaracterized by substantially a same level of experimental control forutilizing substantially the same exposure period for the C controlsamples as for the T test samples.

The embodiment of τ′<τ facilitates a conservative test in which the Ttest samples are exposed to the pressurized gas (including oxygen) for alonger period of time than are the C control samples exposed to thepressurized inert gas. For said embodiment of τ′<τ, if the measuredproperties of the T test samples and the C control sample do notsignificantly differ following the respective exposure periods of τ andτ′, then the test results conservatively show that the exposure of the Ttest samples to the pressurized gas (including oxygen) was negative withrespect to said differences in the measured properties. However, if ameasured property of the T test samples and the C control samplesignificantly differs following the respective exposure periods of τ andτ′, then the test results are indicative of a need for further testingor analysis to differentiate between: 1) the effect of the exposure ofthe T test samples to the pressurized gas (including oxygen); and 2) theeffect of the differential between τ′ and τ.

The embodiment of τ′>τ facilitates an economical test in which the Ttest samples are exposed to the pressurized gas (including oxygen) for ashorter period of time than are the C control samples exposed to thepressurized inert gas. For said embodiment of τ′>τ, if a measuredproperties of the T test samples and the C control sample significantlydiffer following the 0respective exposure periods of τ and τ′ then thetest results show, in an economically short period of exposure, that theexposure of the T test samples to the pressurized gas (including oxygen)causes changes in said measured properties. However, if the measuredproperties of the T test samples and the C control sample do notsignificantly differ following the respective exposure periods of τ andτ′, then the test results are indicative of a need for further testingor analysis to determine whether the exposure of the T test samples tothe pressurized gas (including oxygen) causes changes in said measuredproperties.

Although FIG. 3 shows step 63 being performed after step 62 isperformed, step 63 may alternatively be performed before step 62 isperformed.

Since the inert gas is not expected to chemically react with the moldingcompound, step 63 serves the purpose of being a control step to providea benchmark against which the results of the oxygen pressurization ofstep 62 may be compared. A significant difference between the testsamples and the control samples following execution of steps 62 and 63is suggestive of the possibility that the semiconductor packagingmaterial being tested is unstable in a prolonged exposure to oxygenunder normal operating conditions.

Accordingly after the T test samples and the C control samples have beenexposed, step 64 analyzes the T oxygen-exposed test samples and the Cinert gas-exposed control samples. The analysis of step 64 includes:measuring at least one characteristic common to the C control samplesand the T test samples; and determining whether there exists at leastone significant difference between the at least one characteristic ofthe T test samples and the at least one characteristic of the C controlsamples. If T>1 and/or C>1, which may be equivalently expressed as N≧2,determining whether there exists said at least one significantdifference may comprises performing a statistical analysis of the atleast one characteristic of the C control samples and/or the T testsamples. Step 65 is a decision block which determines the next actionbased on whether said at least one significant difference has beendetermined to exist. If at least one significant difference isdetermined not to exist then the process ends. If at least onesignificant difference not determined to exist then step 66 is nextexecuted, followed by ending the process. Step 66 performs furthertesting, analysis, or testing and analysis of the semiconductorpackaging material to more definitively assess the likelihood of thesemiconductor packaging material being unstable in a prolonged exposureto oxygen. Thus, the method described by the flow chart of FIG. 3 mayserve as a screening process to screen out those semiconductor packagingmaterials which are candidates for being unstable when subjected tocontinuous, prolonged exposure to oxygen under normal operatingconditions.

The measuring in step 64 may include any known measuring technique knownto one of ordinary skill in the art for determining test and controlsample characteristics which, in light of the experimental resultsdiscussed supra, are relevant to assessing the stability of thesemiconductor packaging material. Two such measuring techniques employedby the present inventors are: ion chromatography and thermogravimetricanalysis.

Ion chromatography identifies and quantifies ionic residues on the Ttest samples and the C control samples, as exemplified by the discussionsupra of FIG. 1. Following the exposure of the samples to thepressurized oxygen or to the pressurized inert gas, a quantity ofdeionized water (e.g., 25 milliliters) is added to the vial containingthe samples and the samples are soaked in the ionized water for asufficient period of time (e.g., 24 hours) to collect measurable ionicspecies; then the ionized water is analyzed by ion chromatography todetermine the type and concentration of ions present in the deionizedwater.

Using the ion chromatography, of particular interest forphosphorus-containing molding compounds is the phosphate ion generatedby reaction of phosphorus and oxygen in the presence of water. Asexplained supra in conjunction with FIG. 1, the chloride ion may be ofadditional interest even though the detection of chloride ions may be aconsequence of an oxygen reaction with phosphorus and water such thatchlorine or the chloride ion does not directly participate in a chemicalreaction involving oxygen. Other ions of interest may include, interalia, fluoride ions, sulfate ions, nitrate ions, etc. The particularions of interest depends on the particular packaging material at issue.

In reviewing the ion chromatography data for a significant differencebetween the detected ions of the T test samples and the C controlsamples, the table of FIG. 1 shows such a significant difference in bothphosphate ions and chloride ions for package types 1-5 and nosignificant difference for package types 6-10. Thus based on the ionchromatography results of FIG. 2A, packages 1-5 are candidates for beingunstable when subjected to continuous, prolonged exposure to oxygenunder normal operating conditions, while packages 6-10 are notcandidates for being unstable when subjected to continuous, prolongedexposure to oxygen under normal operating conditions.

Thermogravimetric analysis measures weight changes in each sample as afunction of temperature, as exemplified by the discussion supra of FIGS.2A-2B where the temperature range is 30° C. to 400° C. In FIG. 2A, thethermogravimetric profiles reveal a significant difference between thetest sample that had been subject to oxygen pressurization and thecontrol sample that had been subjected to nitrogen pressurization. Thusbased on the thermogravimetric profiles of FIG. 2A, the packagingmaterial of the test sample in FIG. 2A is a candidate for being unstablewhen subjected to continuous, prolonged exposure to oxygen under normaloperating conditions. In FIG. 2B, the thermogravimetric profiles did notreveal a significant difference between the test sample that had beensubject to oxygen pressurization and the control sample that had beensubjected to nitrogen pressurization. Thus based on thethermogravimetric profiles of FIG. 2B, the packaging material of thetest sample in FIG. 2A is not a candidate for being unstable whensubjected to continuous, prolonged exposure to oxygen under normaloperating conditions.

FIG. 4 depicts a test environment 50 for implementing step 62 or 63 ofFIG. 3. In FIG. 4, the test environment 50 comprising a chamber 10containing semiconductor package samples 11-13 on a support 20,pressurized gas 5, moisture 7, and a heater 30, in accordance withembodiments of the present invention. Alternatively, the samples 11-13may be positioned directly on the bottom surface 22 of the chamber 10instead of on the support 20. The pressure of the gas 5 is controlled bygas inlet line 40 with valve 41 and gas outlet line 44 with valve 45. Ifthe samples 11-13 are test samples, then the gas 5 comprises oxygen asin step 62 of FIG. 3. If the samples 11-13 are control samples, then thegas 5 is an inert gas (e.g., nitrogen, argon, etc.) as in step 62 ofFIG. 3. In FIG. 4, the temperature of the gas in the chamber iscontrolled by the heating jacket 30 or any other means for heating thechamber 10. The moisture 7 may be set at any desired relative humidityby any means or method known to a person of ordinary skill in the artsuch as a source of water within the chamber (not shown), or acombination of water and various salts, vaporized to generate themoisture 7. The chamber 10 may be evacuated by a vacuum pump (notshown). Instrumentation (not shown) for measuring the gas pressure, gastemperature, and relative humidity in the chamber 10 may also bepresent.

As described herein in relation to the present invention, the testingusing pressurized oxygen at elevated temperature accelerates changes insemiconductor packaging material in order to simulate actual changesthat are expected to occur in the semiconductor packaging material underprolonged exposure to oxygen under normal operating conditions. Thetesting and subsequent analysis of the test data can be used to rule outfuture concerns associated with stability of packaging materials in anoxygen environment, or as a screening tool to trigger a need for furtherinvestigation of the stability of the packaging materials in light ofoxidation reaction concerns. Said testing and analysis can beadditionally used to evaluate parts prepared from the same moldingcompound but under different molding conditions. Variations in moldingconditions or in packaging design could result in increased or decreasedfailure rate, even for the same molding compound. Said testing andanalysis can also be used to compare the stability of differentpackaging materials, including different packaging materials havingsimilar or different chemistries, or having been generated by similar ordifferent fabrication methods, or having been packaged or otherwiseformed within its module or package by a similar or different packagingprocess. To improve an evaluation of the probability of failure of thesemiconductor package, said testing and analysis may be used incombination with conventional reliability tests including tests whichinduce mechanical stresses, thermal stresses, excess moisture, etc., andconventional tests for mechanical faults, electrical faults, etc.

While embodiments of the present invention have been described hereinfor purposes of illustration, many modifications and changes will becomeapparent to those skilled in the art. Accordingly, the appended claimsare intended to encompass all such modifications and changes as fallwithin the true spirit and scope of this invention.

1. A test environment, comprising a chamber containing S samples, apressurized gas, and moisture, wherein the S samples each comprise asemiconductor packaging material that comprises one or more cured rawmaterials that include a molding compound, wherein S is at least 1 andif S>1 then the S samples are substantially identical, wherein the Ssamples are being exposed to the pressurized gas and the moisture,wherein the pressurized gas includes at least one of oxygen gas and aninert gas, wherein the pressurized gas has a temperature T satisfyingT_(G)−ΔT₂≦T≦T_(G)−ΔT₁ such that 0<ΔT₁≦ΔT₂ for glass transitiontemperature T_(G) of the molding compound, wherein the moisture has arelative humidity H such that H₁≦H≦H₂, wherein H₁≧20% and H₂≦100%,wherein T_(G)−ΔT₂ is at least about 20° C., wherein if the pressurizedgas includes the oxygen gas then the partial pressure of the oxygen gasis at least about 15 psi, and wherein if the pressurized gas does notinclude the oxygen gas then the pressure of the inert gas is at leastabout 15 psi.
 2. The test environment of claim 1, wherein the chambercomprises instrumentation configured to measure the pressure of thepressurized gas, the temperature T, and the relative humidity H.